With the maturity of automatic sheet feeding technologies, automatic document feeders are widely used in a diversity of document processing machines such as printers, copiers and multifunction peripherals. An automatic document feeder is used for successively feeding a stack of sheet-like documents into the inner portion of the document processing machine so as to implement associated operations and achieve the labor-saving purpose.
For processing sheet-like documents of various sizes, the automatic document feeder has a sheet size detecting mechanism for detecting the size of the document before the document is fed into the inner portion of the document processing machine. Take a copier for example. There are several paper feeding cassettes accommodating blank paper sheets of various sizes (A3, A4, B4, B5, . . . , etc). For example, in a case that an A4-sized document is selected to be copied by the copier, the sheet size detecting mechanism will detect the size of document in advance and issue a corresponding detecting signal to the copier. In response to the detecting signal, an A4-sized blank paper sheet is automatically provided by the copier, and a copying operation is performed on the A4-sized blank paper sheet.
FIG. 1 is a schematic perspective view illustrating a sheet size detecting mechanism disclosed in Taiwanese Patent No. 00588545. FIG. 2 is a schematic top view illustrating a logic plate and a sensing module of the sheet size detecting mechanism as shown in FIG. 1. As shown in FIGS. 1 and 2, the sheet size detecting mechanism comprises a sheet input tray 11, a logic plate 12 and a sensing module 13. The sheet input tray 11 comprises a sheet holder 111 and two opposed sheet guides 112 and 113. The relative location between the sheet guides 112 and 113 is adjustable. As such, the sheet guides 112 and 113 are close to or far from each other to clamp both sides of the paper sheet. The sensing module 13 is fixed under the sheet input tray 11. The logic plate 12 is connected to the sheet guide 113. As such, the logic plate 12 and the sheet guide 113 may be synchronously moved. The logic plate 12 is strip-shaped, and plural featured structures 121 are formed on the surface of the logic plate 12. By moving the sheet guide 113 to comply with the sheet width, a relative motion between the logic plate 12 and the sensing module 13 is rendered and plural digital signals are generated. According to the digital signals, the sheet size is determined. This sheet size detecting mechanism, however, still has some drawbacks. For example, the strip-shaped logic plate 12 is costly. In addition, the use of the logic plate 12 occupies much space of the sheet input tray 11.
For solving the above drawbacks, a sheet size detecting mechanism for saving space of the sheet input tray is disclosed in for example U.S. Pat. No. 6,070,048. FIG. 3 is a schematic perspective view illustrating a sheet size detecting mechanism disclosed in U.S. Pat. No. 6,070,048. As shown in FIG. 3, the sheet size detecting mechanism 4 comprises a sheet guide 41 with racks 411, a circular gear 42 and a variable resistor 43. The racks 411 of the sheet guide 41 are engaged with the circular gear 42. The circular gear 42 is connected with the variable resistor 43. As such, the circular gear 42 and the variable resistor 43 are synchronously rotated. When the sheet guide 41 is moved to comply with the sheet size, the circular gear 42 is correspondingly rotated and thus the variable resistor 43 generates an output resistance value. The output resistance value is varied according to the rotating amount of the circular gear 42. That is, the sheet size may be detected according to the output resistance value. Since the variable resistor 43 is not cost-effective, the sheet size detecting mechanism is still unsatisfied.